Annunciator and paging systems within such facilities as factories, office buildings, parks, schools, and the like can use electrically activated bells as well as speaker-generated tones to announce normal periodic events such as breaks, shift changes, and other non-emergency events. Such systems are commonly limited to a single sound, in the case of those using mechanical bells, and a range of sounds, in the case of those using speakers and driven from a central audio tone source.
Some annunciator system designs use an individual loudspeaker at each of a multiplicity of locations. In some versions, they are wired in parallel, with each speaker transformer-isolated to permit high transmitter signal voltage at low current, which can reduce copper losses. Other designs may use signals sent from a central source at comparatively low levels, with the annunciators equipped with power supplies and amplifiers driven by local AC power. Systems with multiple zones to be signaled at different times or under different circumstances may be directly wired by zone from a shared control panel. Annunciators wired individually back to a control panel may be activated individually using switches. Volume control may be realized using a central attenuator or an attenuator at each speaker.
A logical extension of the speaker system concepts outlined above may be found in existing digital annunciator systems, which can take advantage of the significant flexibility available to digital systems in general to add features not available in earlier designs. Digital designs can include direct addressing of individual annunciators through a signal distribution system, so that a digital communication processor circuit in an individual annunciator can recognize its own address and respond appropriately.
A representative signal distribution system in use employs RS-485, a standard developed by industry and recognized by the Electronics Industry Association (EIA). RS-485 is a two-wire transmission line communication bus that uses a differential serial data stream for communication between one talker at a time and multiple listeners. RS-485 can be configured to be sufficiently flexible to permit each listener to reply when commanded to do so and to permit multiple talkers to talk in turn, using a scheduling protocol to avoid bus contention. The message bits comprising RS-485 may serve as alert signals, address bits, data bits, and checksums, as well as to be assigned other meanings. Commercial off-the-shelf (COTS) integrated circuits and associated circuitry that can be incorporated into annunciators can recognize RS-485 signal traffic, and can be programmed to recognize their own addresses, to interpret commands sent out on the bus, to execute commands, and to take over the bus to transmit a reply when directed to do so.
Annunciators using RS-485 for communication with a central annunciator control panel can be addressed individually using a variety of addressing systems, including for example switch-selected binary code numbers that are transmitted to select each annunciator individually. Some of these annunciators can be commanded to respond to zone messages; subsequently, commands can address these zones instead of individual annunciators, allowing large groups of annunciators to be activated simultaneously. Some designs permit assignment to zones to be established and changed without need to alter physical wiring within a facility.
Annunciator systems in general emit audible tones when activated from a central location. More capable systems may provide the alternative of emitting prerecorded voice announcements or other brief, locally stored recordings. A nominally digital annunciator design in common use further extends this capability by allowing continuous analog output as well as synthesized tones and short messages. For such an annunciator, digital communication with a base station may be augmented with analog signal distribution on a second wire pair, broadcast, typically amplified at the annunciator, and emitted along with or in place of the annunciator's digitally generated tones.
It would be desirable to have an annunciator system with increased capability and flexibility, to take advantage of the opportunities offered by incorporating computer technology into annunciator systems to a greater extent than has been done heretofore.